Method for selecting and installing a dynamic pedicle screw

ABSTRACT

A modular dynamic pedicle screw system including anchoring device having a threaded shank for anchoring within a vertebra, an intermediate element and a head portion configured to receive and secure a rigid or non-rigid stabilization rod. The threaded shank, the intermediate element and the head portion of the anchoring device are cannulated to permit percutaneous implantation of the device. The intermediate portion is designed to be removable from the threaded shank portion subsequent to implantation of the anchoring device to enable substitution of another intermediate element having different dynamic characteristics. The dynamic stabilization system includes an adjustable torque limiting device that is interchangeable between a tap device and a screw driver. The torque device provides information relative to the patient&#39;s bone quality inter-operatively in order to determine the appropriate modulus of elasticity for the dynamic pedicle screw.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Non-Provisionalpatent application Ser. No. 12/336,886, entitled “Modular Pedicle ScrewSystem”, and filed on Dec. 17, 2008, which in turn is acontinuation-in-part of U.S. Non-Provisional patent application Ser. No.12/202,802, entitled “Modular Pedicle Screw System”, and was filed onSep. 2, 2008, the entire contents of which are hereby expresslyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to dynamic spinal stabilization systems.The invention provides a modular pedicle screw attached to the vertebraeto anchor the stabilization system. The modular pedicle screw isconfigured to have dynamic characteristics of varying degree. The systemincludes torque limiting wrench that is interchangeable as either a tapor screw driver.

BACKGROUND OF THE INVENTION

The spine is comprised of an intricate system of bones and assortedtissues that support the body and provides protection of the centralnervous system including the spinal cord and associated nerves. Withinthe spinal column are stacked a plurality of vertebrae separated fromone another by an intervertebral disc that dampens and cushions thecompressive forces exerted upon the spinal column. Located behind theseries of alternating vertebrae and discs is the vertebral canal whichcontains the spinal cord and other associated nerves.

There are more than twenty vertebrae within the spinal column and theyare categorized into one of four classifications: cervical, thoracic,lumbar or sacral. The upper seven vertebrae, including the first sevenextending downward from the base of the skull are referred to as thecervical vertebrae. The next twelve extending downward from the cervicalvertebrae are known as the thoracic vertebrae. Extending downwardly fromthe thoracic vertebrae are the five lumbar vertebrae. At the base of thespinal column is the sacral bone which also includes the coccyx. Thestructural and functional relationship of the vertebrae, discs, muscles,ligaments and nerves enables a healthy normal spinal column to move andarticulate freely almost without limitation.

The spinal column is comprised of the vertebral body, the pedicle, thespinous process, the transverse process, the facet, the laminar arch,and the vertebral canal. The vertebral body is the generallycylindrically shaped weight bearing structure of the vertebra. Thespinous process extends from the rear portion of the vertebra and thetransverse processes extend from each side of each vertebra. Both thespinous process and the transverse process connect muscle tissue andligaments to the spine. The vertebral canal is formed between thevertebral body and the lamina and houses the spinal cord therein. Thepedicle is connected to the vertebral body and supports the lamina.

The spinal column may be subject to numerous abnormalities and disorderswhich can be caused by trauma, disease, or genetic defect such asruptured or slipped discs, degenerative disc disease, fracturedvertebrae as so forth. Such defects can result in conditions causingextreme pain and reduced or abnormal nerve function. These spinalabnormalities can potentially cause damage to the nervous system and inparticular the spinal cord and likewise impair the normal freedom ofmotion of the spinal column.

It is not uncommon to treat such abnormalities surgically by spinalfusion wherein one or more vertebral bodies are fused together. However,spinal fusion may limit the spinal cord's range of motion in rotationand lateral bending. In addition, spinal fusion may increase the stressplaced upon non fused adjacent vertebral bodies thereby diminishingtheir structural integrity. Moreover, the fusion device or material maybecome dislodged and move away from the area of implantation.

A wide variety of approaches have been in use to achieve spinal fusionby implanting artificial devices in or on the spinal column to result inimmobilization. One approach utilizes an anterior implant where theimplant is located on the anterior, or front portion, of the vertebralbody. An anterior stabilization can include full or partial discreplacement by a rigid spacer that is approximately the size of the discthat has been removed. A different approach involves the utilization ofa posterior implant. Posterior implants include rods that are attachedto either the lamina or transverse process by hooks or by pediclescrews. Other posterior implants allow for flexible or dynamicstabilization using pedicle screws connected by rigid or flexible rodmember. Prior art posterior pedicle screw based stabilization systemscreate forces that are often transferred to the anchored pedicle screws.Patients having a relatively brittle bone structure cannot withstand themagnitude of these forces without resulting in the failure of theanchoring system.

DESCRIPTION OF THE PRIOR ART

One example of a dynamic anchoring device is disclosed in US PatentApplication Publication 2004/0025289 by Biedermann et al. The deviceincludes an element for anchoring in a bone or vertebra and a headconnected to the shank, a receiving part for receiving the head, and apressure element acting on the head, wherein the pressure element isresilient so that upon a movement of the element from a first angularposition of the shank relative to said receiving part into a secondangular position the pressure element exerts a return force onto thehead to urge the element towards the first angular position.

Another example of a dynamic anchoring device is disclosed in US PatentApplication Publication 2005/014823 to Boyd et al. The dynamicstabilization system disclosed therein includes bone anchors having aflexible portion between the bone engaging and head portions of theanchor.

U.S. Patent Application Publication 2005/0216003 to Biedermann et aldiscloses a bone anchoring element such as a screw. The screw has ashaft and a first head. A second head is elastically connected to thefirst head. The second head is arranged in the receiving member suchthat the second head can pivot or swivel. The second head is fixed inthe resting member in an angular resting position. The screw isdeflectable from the angular head position relative to the second head.The second head is elastically connected to the first head such that arestoring force returns the screw to the angular resting position. Theresting angular position of the shaft relative to the receiving part isadjustable.

U.S. Patent Application Publication 2006/0129147 to Biedermann et aldiscloses a stabilization device for bones or vertebrae that comprises asubstantially cylindrical elastic element. The elastic element has afirst end and a second end opposite to the first end. An elastic sectionextends between the first end and the second end. The elastic sectionincludes at least first and second helical coils. The first and secondhelical coils are arranged coaxially so that the first helical coilextends at least in a portion between the second helical coil. Theelastic element may form, for example, a portion of a rod, boneanchoring element, or plate.

U.S. Patent Application Publication 2007/0055236 to Hudgins et aldiscloses an apparatus and method for stabilizing the facet joints ofthe spine. The facet implant may be in the form of a screw or otheranchor with the intermediate portion in the form of a polyaxial head, acord a spring, etc.

Another device for the dynamic fixation of impaired spinal columnsegments in disclosed in U.S. Published Patent Application 2007/0233087to Schlapfer. The device includes an intermediate element for adetachable, lockable, ball joint like connection having an outer wallconcentric with the longitudinal axis and an inner wall forming acoaxial cavity. Either the outer wall or the inner wall comprises one oftwo contact zones that form the ball joint like connection. Theintermediate element is at least partly made of a super elasticmaterial.

U.S. Published Patent Application 2008/0021465 to Shadduck et aldiscloses a spine implant device for fusion or dynamic stabilization ofa spine segment that includes a fixation device with a shaft portion forengaging bone and a proximal end for coupling to a rod that allows forlimited flexing of the proximal end relative to the shaft portion.

A further example of a dynamic spinal stabilization system is disclosedin US Published Patent Application 2008/0071273 to Hawkes et al.Disclosed is a system for stabilizing at least one spinal motion segmentthat includes a fastener having an anchoring portion and a couplingportion and a longitudinal support member couple to the fastener whereina portion of the system is formed from a super-elastic material.

U.S. Pat. No. 7,363,838 to Abdelgany discloses a method and assembly fortightening a locking element in an orthopedic implant. The assembly iscomprised of a ratcheting mechanism that includes a shaft portion and asleeve portion operatively connected to the shaft portion. The assemblyfurther includes a first handle or wrench operatively connected to thesleeve portion.

U.S. Pat. No. 5,437,524 to Huang discloses a torque adjustmentcontroller for use with a tool to safely adjust and control the torquethe tool works on a workpiece. The torque adjustment controller mainlyconsists of a housing, an output shaft, an input shaft, a stepped ring,a lugged moveable member contacting the stepped ring, a torque springdisposed between the output and the input shafts. The adjusted torque istransmitted from the input shaft to the output shaft to work on theworkpiece to safely protect the latter from being damaged due toimproper torque applied by the tool.

U.S. Pat. No. 5,626,474 to Kukla et al discloses a manually operateddental implant torque wrench that includes a drive assembly. The driveassembly includes a receptacle end rotatably mounted to the second openend and adapted for attaching a dental tool thereto. An adjustabletorque limiting assembly is connected to an elongated shaft fordisengaging the elongated shaft assembly from the rotation of a driveassembly when rotation of the elongated shaft assembly has reached anadjustable predetermined torque setting.

U.S. Pat. No. 6,330,845 to Meulink discloses a wrench that guardsagainst displacing an implant or splitting a bone. The wrench assemblyincludes a handle and a socket shaft depending from the handle in atorque transmitting relation. The socket has an implant engaging portionfor engaging the implant to torque transmitting relation. A torquewrench is engageable with a drive shaft to facilitate applying a knowntorque to the implant. The torque wrench has a handle and a torqueindicator responsive to the flexing of the handle to indicate the amountof torque being generated at the engagement end of the torque wrench.

SUMMARY OF THE INVENTION

The present invention relates to a spinal stabilization system thatprovides for dynamic stabilization using a modular screw in conjunctionwith a rigid or non-rigid rod that permits load transfer at the pediclescrew rod interface as opposed to the dynamic rod per se. The screw hasan elastic segment interposed between a threaded portion of the screwand the screw head portion, also referred to as a “tulip”. The amount ordegree of motion can be varied based on the rigidity or flexibility ofthe elastic material as well as the length and diameter of the elasticmaterial. The pedicle screw is designed to be used in a percutaneousdynamic spinal stabilization system. The screw can be used in a singleor multi-level construct in combination with a titanium, PEEK or Nitinolrod. The dynamic screw design enables percutaneous delivery of thestabilization system although the dynamic system can be used in an openapplication as well.

The dynamic spinal stabilization system includes a dynamic modularpedicle screw system which in turn preserves motion in the posteriorcolumn of the human spine. The dynamic screws can be used in conjunctionwith a rigid or non-rigid rod. The dynamic pedicle screw used with arigid rod will allow for the load transfer to occur at the screw/rodinterface as opposed to a non-physiologic load transferred through adynamic rod alone. Alternatively, the modular pedicle screw can includea rigid segment interposed between a threaded portion of the screw andthe screw head portion, also referred to as a “tulip”.

The dynamic stabilization system includes an adjustable torque limitingdevice that is interchangeable between a tap device and a screw driver.The device is initially used as a tap which gives an initial indicationof screw insertional torque. This initial indication provides the basisfor selecting the dynamic characteristic of the pedicle screw. Thedevice is additionally used as a screw driver which confirms the initialindication during the screw insertion process. This torques limitingdevice enables a surgeon to determine in-situ patient informationrelative to bone quality. This information is required in order todetermine the appropriate modulus of elasticity of the intermediatecomponent of the dynamic pedicle screw.

Accordingly, it is an objective of the instant invention to provide asemi dynamic spinal stabilization system that allows for variablecustomization of the elastic member thereby increasing the ability tospecifically address a greater number of pathologies.

It is also an objective of the instant invention to provide a torquelimiting device that can interchangeably function as either a tap orscrew driver to thereby provide the surgeon with information regardingthe in-situ bone quality such that a pedicle screw with an appropriatedynamic characteristic can be selected.

It is a further objective of the instant invention to provide absorptionof the dynamic force transmission within the anchoring screw and not atthe bone-screw interface.

It is yet another objective of the instant invention to provide amodular pedicle screw that is designed to be used in a percutaneousdynamic stabilization system.

It is a still further objective of the invention to provide a kit ofmodular anchoring devices for a dynamic spinal stabilization system. Theanchoring device is a three part design including a threaded rigidshank, an intermediate component that is an elastic polymer or rigidmaterial, and a rigid multi-axial tulip. The kit would include aplurality of threaded shanks of varying sizes, a plurality ofintermediate portions of varying geometries and rigidities, a pluralityof tulip heads and a torque related instrument that can interchangeablyfunction as either a tap or screw driver.

It is a further object of the invention to provide an intermediatecomponent that is designed to be removable from the threaded shankportion subsequent to implantation of the pedicle screw should thepathology change thereby necessitating a change in the flexibility ofthe dynamic system. The ability to change the dynamism of thestabilization system without removing the threaded shank portion allowsthe surgeon to maintain the original bone purchase in the patient whichfacilitates the procedure, the healing process and improves thepotential for long term success.

Another distinct objective of the system is to provide a morecomprehensive yet less invasive method to address more complex spinecases, i.e. spinal deformity cases. Currently, dynamic systems arelimited in their applicability and mostly ruled out for use in morecomplex spine cases. One reason may be due to the limited ability tomanipulate the individual spine segments in order to obtain the overallcorrection/objective. This reinforces a current perception that a moreinvasive technique is always required. This system may not be applicablein all complex cases however it will be a minimallyinvasive/percutaneous dynamic screw option for surgeons to consider.

Other objectives and advantages of this invention will become apparentfrom the following description taken in conjunction with anyaccompanying drawings wherein are set forth, by way of illustration andexample, certain embodiments of this invention. Any drawings containedherein constitute a part of this specification and include exemplaryembodiments of the present invention and illustrate various objects andfeatures thereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective disassembled view of the dynamic modular pediclescrew.

FIG. 2 is a perspective view of the dynamic spine stabilization systemshowing a multi level construction utilizing a pair of dynamic pediclescrews and a stabilization rod.

FIG. 3A is a top view of the elastic intermediate member.

FIG. 3B is a side view of the elastic intermediate member.

FIG. 3C is a sectional perspective view of the elastic intermediate.

FIG. 4A is a top view of a second embodiment for the elasticintermediate member.

FIG. 4B is a side view of the second embodiment for the elasticintermediate member.

FIG. 4C is a sectional perspective view of the second embodiment for theelastic intermediate member.

FIGS. 5A and 5B show various configurations for the elastic portion andtheir relative dynamic properties.

FIGS. 6A and 6B show a third and fourth embodiment for the elasticintermediate member.

FIG. 7A is an exploded side view of the lower coupling the intermediatemember and the threaded shank.

FIG. 7B is a top view of the lower coupling member of the intermediatemember.

FIG. 7C is a top of view of the threaded shank portion.

FIG. 8A is a side view of the upper coupling member and the tulip head.

FIG. 8B is a top view of the tulip head component.

FIG. 8C is a top view of the upper coupling member of the intermediateelement.

FIG. 9 is a perspective view of the torque limiting device with a tapinserted in the driver.

FIG. 10 is a perspective view of a screw driver shaft that can beselectively substituted for the tap shaft shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a dissembled view of the modular dynamic pedicle screw 1.Screw 1 includes a threaded shank portion 2 having a one end that tapersinto a point 4 at one end and has an opposite end 6 that includes acoupling element 8. Coupling element 8 includes internal female threads10. The pedicle screw 1 has a channel 20 through the entire length ofthe pedicle screw, including the tulip head 12, the intermediatecomponent 14, and the threaded shank portion 2. This channel 20 allowsthe pedicle screw 1 to be maneuvered on a Kirschner wire 22, also knowas a K-wire. In practice the K-wire is positioned within the patientusing fluoroscopy, or other imaging techniques, so as to provide precisepositioning of the pedicle screw 1. Once the components are securelypositioned the K-wire can be easily removed through the channel 20 whichis open at the end of the threaded shank portion and extends through theuppermost portion of the head portion or tulip 12. The threaded shank 2is externally threaded. The threads 3 can be fenestrated or partiallyfenestrated. Fenestrated threads are particularly appropriate forosteoporotic patients or patients who require greater assurance ofincreased pedicle screw purchase based on bone quality. The threadedshank 2 of the pedicle 1 are appropriately sized in relation to thepatient's pathology and can be formed in different lengths and externalthreaded diameters.

The head or tulip portion 12 of the pedicle screw 1 includes upwardlyextending cylindrical wall 15 wherein grooves 17 are positioned indiametrically opposed relationship. These opposing grooves 18 allow fortop loading of either a rigid or non rigid rod 30 into the tulip. Thetulip may be fixed or multi axial. The inner portion of the cylindricalwall accepts a threaded lock screw 32 to secure the rod 30 to thepedicle screw 1. The tulip design can accept tulip extension towers,attached to tulip portion 12, which will facilitate the percutaneouspassing of the rod 30 through multiple screws based upon the number ofspinal segments involved in the overall dynamic spinal stabilizationsystem. The tulip extensions allow for external control of the tuliphead during the rod delivery process. The screw extensions that areattached to the tulip portions remain in place until the percutaneousdelivery and placement of the rod 30 has been achieved and threaded lockscrews 32 have been finally tightened. In addition, the pedicle screw 1is also configured to receive a shank extension tower. The screwextension tower is a completely rigid device that extends dorsallythrough the skin incision. This feature enables three dimensionalmanipulation of the spine segment. Once the rigid manipulation of thesegment is complete the screw extension tower is removed and the dynamicmember is fully functional. The tulip design allows for top loading ofthe rod 30 delivered under direct visualization as is possible when thesurgery is performed under open conditions. A coupling element 21 havinga cylindrical wall with external threads for engagement with theintermediate member 14 is attached to tulip portion 12 with a ball andsocket arrangement 24.

The intermediate portion 14 of the dynamic pedicle screw includes anelastic portion 40, an upper coupling member 16 and a lower couplingmember 18. As shown in FIGS. 3A-3C, portion 40 is generally cylindricalin shape and includes a passageway 42 concentric with the longitudinalaxis of the cylindrical body. The portion 40 is formed from elasticmotion preserving dynamic material which allows for the requisite degreeof motion and is capable of standing the mechanical loads associatedwith the human spine. This provides intraoperative flexibility for thesurgeon to choose or customize the construct to address the patient'sspecific pathology. The portion 40 is available in varying levels,ranges and modes of dynamism, such as dynamic, motion preserving,non-fusion and rigid. Dynamism can be adjusted based on the type ofmaterial used, for example Nitinol or polycarbonate, the length of thecylinder, the diameter and or wall thickness of the cylinder or anycombination of the above variables (as shown in FIGS. 5A and 5B).Embedded within the wall of cylindrical portion 40 is a jacket 44 madefrom a polyester material, or the like, which extends outwardly fromeach end of the cylinder 40, as shown in FIGS. 3A through 3C. A secondembodiment, shown in FIGS. 4A through 4C utilizes a polyester, or thelike, jacket that surrounds the outer surfaces of cylindrical member 40and extends outwardly from each end of the cylinder 40. Extendingportions 48 of the jacket extend into tabs formed in the upper and lowercoupling members, 16 and 18 respectively, to complete the assembly ofthe intermediate portion 14. Upper coupling member 16 includes acylindrical wall having an externally threaded surface. Upper couplingmember 16 is threadably connected to tulip coupling member 21. Likewise,lower coupling member 18 includes a cylindrical wall having anexternally threaded surface. Lower coupling member 18 is screwed on tocoupling member 8 positioned on the threaded shank portion 2. As analternative, cylindrical member 40 can be bonded, glued or moldeddirectly on to the upper and lower coupling members, 16 and 18respectively, without the utilization of a jacket.

The intermediate portion can also be rigid allowing for rigid fixation.In order to assemble a rigid modular screw a non-elastic intermediateportion 14 is coupled to the threaded shank portion 2 and the tulip headportion 12. In this instance, cylindrical member 40 can be made from thesame material as the threaded shank 2 or the tulip head 12 or some otherrigid compatible material. The non-elastic cylindrical member 40 can bethreaded into upper and lower coupling members or otherwise suitablyaffixed thereto.

FIGS. 6A and 6B illustrate a third and fourth embodiment for theintermediate member 54. As shown in FIG. 6A intermediate member 54includes an upper coupling member 56 that includes a threaded portion 57which is sized and configured to threadably connect to tulip couplingmember 21. Upper coupling member 54 is generally cylindrical in shape.It includes an upper cylindrical portion 51 adjacent the threadedportion 57 having a first diameter. Depending downward from the uppercylindrical portion is a post like cylindrical portion 53 having acenter coincident with the upper cylindrical portion diameter 51.Depending downward from the post like cylindrical portion 53 is aninterengaging cylindrical portion 55 whose center is coincident withboth the upper cylindrical portion 56 and the post like cylindricalportion 53. The diameter of the interengaging cylindrical portion 55 isgreater that the post like cylindrical portion 53 but less than theupper cylindrical portion 51. Intermediate member 54 also includes alower coupling member 58 having a threaded portion that is sized andconfigured to threadably engage threads 10 on coupling member 8. Thelower coupling member 58 has a lower cylindrical portion 57 having adiameter substantially the same size as the upper cylindrical portion 51of the upper coupling member 56. Extending upwardly from the lowercylindrical member is a hollow cylindrical wall 70. The upper portion ofthe hollow cylindrical wall terminates in an annular flange 72 thatextends radially inward to form a cylindrical cavity having a reduceddiameter aperture. The diameter of the aperture is sufficiently large toallow the interengaging cylindrical portion 55 to pass there throughwhen introduced at an appropriate angle. Once the upper and lowercoupling members are properly positioned, with the interengagementcylinder 55 of the upper coupling member 56 located within thecylindrical cavity of the lower coupling member 58, a synthetic material50, such as a polycarbonate urethane, is injected into the space formedbetween the upper and lower coupling members. The modulus of elasticityof the injection molded material 50 is variable and can provide a rangeof stiffness from rigid to flexible. Likewise, the lengths and diametersof the upper and lower coupling members can be changed to allow forvarying amounts of synthetic material 50 to be injected between the twomembers. By varying the length, diameter, or wall thickness of syntheticmaterial 50 the degree of elasticity of intermediate member 54 can bevaried. The synthetic material can be appropriately color coded, and orotherwise marked with indicia, to provide a visual indication of theelasticity of the injection molded material. The surfaces of the upperand lower coupling elements are properly surface treated prior toinjection of the synthetic material to provide an optimum amount ofadhesion between the synthetic material and the upper and lower couplingmembers. The assembled intermediate member 54, including the upper andlower coupling members and the synthetic material 50 is designed tohandle a torque in the range of 80 to 120 inch pounds of force. Inaddition the intermediate member provides five degrees of motion,including flexion/extension and is capable of handling force in theorder of 250 to 400 newtons. The upper and lower coupling members 56 and58 are made from titanium or any other suitable biocompatible material,either metallic or synthetic. All surface edges of the upper and lowercoupling members are rounded to remove sharp surface edges from theintermediate member.

The embodiment shown in FIG. 6B is similar to that shown in FIG. 6Aexcept that in this embodiment upper flanges 62 and lower flanges 64 aresubstituted for the threaded portions 57 and 59 respectively. Flanges 62and 64 include two or more spaced flange segments (62A, 62B and 64A,64B) that cooperate with complimentary recesses and grooves formed onthe tulip coupling member 21 and the threaded shank member 2.

FIG. 7A illustrates the lower coupling member 18 and the threaded shank2 with its coupling member 8. Annular threads 10 on coupling 8 mate withexternal threads on lower coupling member 18. FIG. 7B is a top view oflower coupling member 18 showing a socket 19 that includes a portion ofchannel 20. Socket 19 is designed to operatively engage an insertion orremoval tool which can be inserted through the intermediate portion 14via channel 20. Should it be necessary to change the dynamiccharacteristics of the spinal support system the surgeon would removethe rod 30 from the head 12 by first removing threaded lock screw 32.Following removal of the rod 30 the head portion 12 would be unthreadedfrom the intermediate portion 14 using an appropriate tool. Thereafter,a tool would be inserted through the channel 20 in the intermediatemember 14 to engage socket 19 formed in lower coupling member 18. Uponrotation of the tool the lower coupling 18 of the intermediate memberwill be unthreaded from the second coupling 8 formed on the threadedshank portion 2. The intermediate portion can then be removed from thepatient. A new intermediate portion 14 can then be positioned over theexisting threaded shank portion 2. Thereafter a tool would be insertedthrough channel 20 of the intermediate member 14 and engage socket 19formed in the lower coupling member 8. Upon rotation of the tool thelower coupling 18 of the intermediate member 14 will be threaded intothe second coupling formed on the treaded shank 2. The head portion 12can then be threaded onto the intermediate portion 14 and the rod 30 canbe affixed thereto by locking screw 32. The ability to change thedynamism of the stabilization system without removing the threaded shankportion allows the surgeon to maintain the original bone purchase in thepatient which facilitates the procedure, the healing process andimproves the potential for long term success. FIG. 7C is a top view ofthe threaded shank 2 with channel 20 and coupling threads 10.

FIG. 8A is a side view showing tulip head member 12 with cylindricalside walls 15 and groove 17. A coupling element 21 having a cylindricalwall with external threads for engagement with the intermediate member14 is attached to tulip portion 12 with a ball and socket arrangement24. FIG. 8B is a side view of tulip head member 12. FIG. 8C is a topview of the upper coupling member 16.

The rod 30 connects multiple screws based upon the number of segmentsinvolved in the overall construct. The rod can be of any compatiblematerial (PEEK, Titanium, Nitinol, etc). This also increases theversatility of the system allowing for more control in defining therigidity or dynamism of the overall construct. The rod 30 used inconjunction with the dynamic pedicle screw system can be either rigid ornon rigid.

FIG. 9 is a perspective view of the torque device 70 that can beinterchangeably used as either a tap device or screw driver device. Thetorque device includes a “T” shaped handle 72. Handle 72 is mechanicallyconnected to socket 74 through adjustable clutch mechanism 76. Clutchmechanism 76 includes a collar that can selectively set the slip pointof an over load clutch. The set point can be incrementally varied from10 inch-pounds through 80 in-pounds in 10 inch pound increments. A tapdevice 78 can be removeably inserted into socket 74. Tap device 78includes a tap shaft 80 and a plurality of cutting threads 82.

FIG. 10 is a perspective view of the screw driver device 84 that isinterchangeable with the tap device 78 of FIG. 9. Screw driver device 84is likewise removeably inserted into socket 74. Screwdriver device 84includes a screw driver shaft 86 and a screw driver head 88.

As noted, the torque device includes an interchangeable tap device 78 aswell as a screw driver 84. The dynamic stabilization system includes anadjustable torque device 70 that is interchangeable between a tap device78 and a screw driver 84. The device is initially used as a tap whichgives an initial indication of screw insertional torque. This initialindication provides the basis for selecting the dynamic characteristicof the pedicle screw. The device is additionally used as a screw driverwhich confirms the initial indication during the screw insertionprocess. Initially the tap is advanced in a measurable fashion. Startingwith a low torque setting, for example 10 inch-lbs, on control collar 76the tap device 78 is advanced until the overload clutch mechanism 76slips or until the appropriate distance of advance through the pedicleinto the vertebral body has been achieved which ever occurs first. Thevisual confirmation of depth is achieved through the use ofinteroperative fluoroscopy as well as depth markings on the tap shaft80. Should the clutch mechanism 76 slip prior to the tap device 78reaching the desired depth the control collar 76 on the device would berotated to the next higher setting, such as 20 inch-pounds. The processwould be repeated, and the torque setting would be progressivelyincreased until the tap threads 82 have reach the desired depth. Thedevice 70, including handle 72, clutch mechanism 76, socket 74, tapdevice 78 and screw driver 84 are cannulated in design in order toaccommodate percutaneous pedicle preparation and screw delivery. The useof the torque device 70 therefore seeks to maximize the dampening effectand minimize the potential of a loosening side-effect.

The utilization of torque device 70 is necessary for optimum selectionof the appropriate dynamic screw of any configuration and represents anovel technique with the field of spinal instrumentation. The objectiveis to provide information relative to the patient's bone qualityinter-operatively in order to determine the appropriate modulus ofelasticity for the dynamic pedicle screw. For example, if the patient'spedicle tap torque is 40 inch-pounds verses 80 inch-pounds, the patientshould receive a less stiff (lower modulus of elasticity) intermediatecomponent in order to transfer load more appropriately or reduce thestress at the bone/screw interface. The ability to provide the patientwith varying degrees of physiological dampening/stiffening via themodular aspect of the intermediate component necessitates the ability tohave inter-operative determination of the patient's bone quality, orpull out strength, in order to make the most appropriate decision forthe patient.

The appropriately selected screw should have dynamic characteristicsthat should absorb the strain within the implant during and particularcycle and should limit any strain transduction to the bone screwinterface. If the modulus of elasticity of the screw is too high, thescrew will have a higher incidence of loosening. On the other hand, ifthe screw has a modulus that is too low, the screw will not create thedesired effect of physiologic dampening and strain.

The utilization of a torque measuring device such as device 70 isparticularly important in matching the dynamic characteristics of thepedicle screw not only to the patient but to the specific level as itrelates to the overall bone density and fixation requirements of thepatient.

Various types and sizes of the components, namely the intermediatemembers, the threaded shanks, the tulip heads and rods, etc. areindividually wrapped and terminally sterilized. They are brought to theoperating room as a kit and individually selected by the surgeon basedon the case presented to them by the patient. Once the sterilizedpackage is opened the device contained therein is either used ordiscarded. The components can not be sterilized. The kit also includes atorque limiting wrench that is interchangeable as either a tap or screwdriver. The torque limiting wrench including the socket clutch andhandle assembly can be sterilized and is therefore reusable however thetap shaft and screw driver attachments may be disposable.

All patents and publications mentioned in this specification areindicative of the levels of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual publication wasspecifically and individually indicated to be incorporated by reference.

It is to be understood that while a certain form of the invention isillustrated, it is not to be limited to the specific form or arrangementherein described and shown. It will be apparent to those skilled in theart that various changes may be made without departing from the scope ofthe invention and the invention is not to be considered limited to whatis shown and described in the specification and any drawings/figuresincluded herein.

One skilled in the art will readily appreciate that the presentinvention is well adapted to carry out the objectives and obtain theends and advantages mentioned, as well as those inherent therein. Theembodiments, methods, procedures and techniques described herein arepresently representative of the preferred embodiments, are intended tobe exemplary and are not intended as limitations on the scope. Changestherein and other uses will occur to those skilled in the art which areencompassed within the spirit of the invention and are defined by thescope of the appended claims. Although the invention has been describedin connection with specific preferred embodiments, it should beunderstood that the invention as claimed should not be unduly limited tosuch specific embodiments. Indeed, various modifications of thedescribed modes for carrying out the invention which are obvious tothose skilled in the art are intended to be within the scope of thefollowing claims.

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 17. (canceled)
 18. (canceled)
 19. A method for selectingand installing a dynamic pedicle screw comprising the steps of: applyingtorque to a handle of a torque drive device to impart torque thereto;connecting said handle to a clutching mechanism having a first andsecond end, said handle operatively connected to said handle at saidfirst end of said clutching mechanism; connecting a socket member tosaid second end of said clutching mechanism; said clutching mechanismincluding an adjustment member to selectively vary a torque settingwherein said clutch mechanism will slip and the torque entering thefirst end of the clutch mechanism will not be transmitted to the secondend of the clutch mechanism; attaching a tap device having tap threadsinto the socket member and positioning said threads adjacent a hole tobe tapped; setting said adjustment member at a low torque setting andadvancing said tap device; applying torque to said handle until theclutch member slips or said tap device has advanced an appropriatedistance; if said tap device has not reached the appropriate distanceincreasing said torque setting on said adjustment member atincrementally higher values until the tap device has been advanced tosaid appropriate distance; selecting a dynamic pedicle screw with aparticular modulus of elasticity based upon the final torque settingutilized in advancing the tap device to the appropriate distance. 20.The method for selecting and installing a dynamic pedicle screw setforth in claim 19 further including the additional steps of: positioningthe dynamic pedicle screw adjacent said tapped hole; removing said tapdevice from said socket member; attaching a screw driving device havinga screw driving head into said socket member; positioning said screwdriving head into operative engagement with said dynamic pedicle screw;setting said adjustment member at a low torque setting and advancingsaid screw driving device; applying torque to said handle until theclutch member slips or said screw driving device has advanced anappropriate distance; if said screw driving device has not reached theappropriate distance, increasing said torque setting on said adjustmentmember at incrementally higher values until the tap device has beenadvanced to said appropriate distance; and confirming the selection ofsaid dynamic pedicle screw with a particular modulus of elasticity basedupon the final torque setting utilized in advancing the screw drivingdevice to the appropriate distance.